A horn from a lesser demon used for a god emissary job.
Demon horns are used in the emissaries activity. They are collected as part of the fifth job in the combat tree for which you need an Attack, Magic or Ranged level of 50. They are dropped by demons.
Five jogre talismans, a demon horn and a perfect blue dragon scale should be returned to your chosen faction's emissary to complete the job and receive 10,000 XP in any combat skill where the amount granted is no more than a third of the amount you already have in that skill.